Replace execInWindow with execInThread (#6885)
* Revert "Fix freeze when pressing enter to dismiss a dialog while focused in a text box in Mozilla applications."
This changes execInWindow to again use PostMessage rather than SendMessageCallback.
This reverts commit 657fb56a2dfee855cb998ed0903d60d33866079a.
* Replace nvdaHelperRemote's execInWindow with execInThread, which uses PostThreadMessage rather than PostMessage. This still allows us to execute code in a thread via the message queue (avoiding Gecko E10s issues with SendMessage) but also still fixes #6422 as the message will still be received even if the window is destroyed.
